perf(codegen): reduce checks printing strings

[overlookmotel]

Speed up printing strings.

I had wrongly assumed that bytes.next().unwrap_unchecked() would skip checks, but it seems it behaves the same as bytes.next() - i.e. check iterator is not empty, and only advance if it's not.

Use assert_unchecked! instead to inform compiler that there are definitely the required number of bytes remaining in bytes iterator before consuming them.

This reduces instruction count considerably, and removes a branch on every byte consumed.

https://godbolt.org/z/TWzfK1eKj

The mysteries of the compiler!

[overlookmotel]

• perf(codegen): reduce checks printing strings #10341 👈 (View in Graphite)
• main

---

How to use the Graphite Merge Queue

Add either label to this PR to merge it via the merge queue:

• 0-merge - adds this PR to the back of the merge queue
• hotfix - for urgent hot fixes, skip the queue and merge this PR next

You must have a Graphite account in order to use the merge queue. Sign up using this link.

An organization admin has enabled the Graphite Merge Queue in this repository.

Please do not merge from GitHub as this will restart CI on PRs being processed by the merge queue.

This stack of pull requests is managed by Graphite. Learn more about stacking.

[codspeed-hq]

CodSpeed Instrumentation Performance Report

Merging #10341 will not alter performance

_{Comparing 04-09-perf_codegen_reduce_checks_printing_strings (426f8cb) with main (0370363)}

Summary

✅ 36 untouched benchmarks

[overlookmotel]

Tiny effect on benchmark, because strings are a small part of AST, and this change probably only affects printing escapes. In the hot loop where no escape required, compiler probably already deduced there was definitely a byte to consume, because we just peeked it a few instructions earlier.

[overlookmotel]

Tiny optimization, so merging without review.

[overlookmotel]

Merge activity

• Apr 9, 2:53 PM EDT: The merge label '0-merge' was detected. This PR will be added to the Graphite merge queue once it meets the requirements.
• Apr 9, 2:53 PM EDT: overlookmotel added this pull request to the Graphite merge queue.
• Apr 9, 3:00 PM EDT: Merged by the Graphite merge queue.